egui_extras: Make serde an optional feature (#4641)

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All the other crates in egui have serde as an optional dependency -
which is great! But sadly egui_extras unconditionally includes it, which
adds a bunch of code to stuff that may not care for it. This PR gates
serde support behind a new `serde` feature.

This is a breaking change; if that's undesirable then we can add it as a
default feature instead, though that wouldn't match any of the other
crates.

tests/test_ui_stack/Cargo.toml

@@ -19,7 +19,7 @@ eframe = { workspace = true, features = [
 ] }
 
 # For image support:
-egui_extras = { workspace = true, features = ["default", "image"] }
+egui_extras = { workspace = true, features = ["default", "image", "serde"] }
 
 env_logger = { version = "0.10", default-features = false, features = [
   "auto-color",